Voice radio transmissions do not always reach the intended recipient. Voice messages may not reach a recipient for a number of reasons. Messages can interfere with one another when two different senders desire to speak to a recipient simultaneously. While modern radio communication systems allow multiple transmissions in parallel, receipt and listening are serial processes because a human can listen to only one message at a time. One message therefore gets stepped on; that is, transmission may occur, but receipt does not.
Consequences can be severe if an emergency message is stepped on in this manner. The sender might repeat the message until it is received, but in an emergency any delay is undesirable and might be critical. Also, if the sender is experiencing the emergency, it might not be possible to repeat the message and needed assistance would not be rendered.
Stepped-on messages that do not pertain to emergencies may nonetheless have a high priority. For example, a military commander may need to redirect a subordinate unit's movement at the same time a sub unit commander is communicating now-obsolete movement orders to his troops. The higher level commander should have priority, but his messages may be stepped on by lower level communications.
Miscommunication due to voice radio communications being stepped on has not been identified as a historical source of fatalities in commercial aviation. However, it is commonly cited in safety-related aviation incidents (e.g., NASA's Aviation Safety Reporting System) as a reason for missed or confused clearances (or clearance readbacks) and resulting mistakes (e.g., missed heading changes and altitude busts). This is especially true for surface operations at busy airports, particularly when the part of the voice message stepped on is the aircraft call sign.
If the recipient is busy when a message comes in, he or she may not be able to attend to it at once. In a single pilot aircraft, for example, the recipient has numerous other tasks in addition to communication. Existing systems require the recipient to remember the sender's identity and to issue a request to “Say again” later on, which adds a memory task at a time when workload is already high. The added memory task may slow or otherwise degrade performance of more important tasks, or the recipient may forget that a message came in and the communication may never be completed.
During periods of very high workload the recipient may experience attention tunneling (intense focus on one task to the exclusion of others). In this situation the recipient may entirely fail to notice that a message has arrived.
There have been a number of attempts to solve the problems mentioned above. Military personnel, commercial pilots, air traffic controllers, and other radio operators are trained to use specialized language and procedures to maximize communication and minimize interference. Basic notification of specific emergencies can often be sent using other systems besides voice radio, such as special transponder codes, marine emergency beacons or signal flares. Implementation of these solutions requires training of the users of these systems, but training costs both time and money. Training can be imperfect, either because the syllabus is imperfectly designed or because it was improperly applied. Trained-on procedures may be forgotten in emergency situations, especially if such situations are rare (e.g. commercial airline operations). Although training seeks to optimize the use of inherent human resources, it does not increase the amount of a resource (for example, memory of past transmissions) available. Finally, the problems cited above occur today despite training aimed at minimizing them.
Transponders communicate the fact and sometimes the type of an emergency (e.g., 7500 for hijack, 7600 for lost communications) but not its specific nature or details. Similarly, emergency beacons are often limited to transmitting a specific block of information. In contrast, voice radio can transmit any information that the sender possesses. Transponders and beacons also require a second transmission system for both sender and receiver. However, 1) such devices may be able to transmit information more rapidly than voice radio, and may do so automatically; and 2) it is good design practice to use dissimilar redundant systems for critical functions. For these reasons, a voice radio system would complement rather than replace existing emergency notification systems.